Re: [PATCH slab v5 5/6] slab: Reuse first bit for OBJEXTS_ALLOC_FAIL

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Sep 12, 2025 at 5:33 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Fri, Sep 12, 2025 at 05:07:59PM -0700, Alexei Starovoitov wrote:
> > On Fri, Sep 12, 2025 at 5:02 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> > > On Fri, Sep 12, 2025 at 02:59:08PM -0700, Alexei Starovoitov wrote:
> > > > On Fri, Sep 12, 2025 at 2:44 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > > >
> > > > > On Fri, Sep 12, 2025 at 02:31:47PM -0700, Alexei Starovoitov wrote:
> > > > > > On Fri, Sep 12, 2025 at 2:29 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > > > > >
> > > > > > > On Fri, Sep 12, 2025 at 02:24:26PM -0700, Alexei Starovoitov wrote:
> > > > > > > > On Fri, Sep 12, 2025 at 2:03 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > > > > > > > >
> > > > > > > > > On Fri, Sep 12, 2025 at 12:27 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > > > > > > > >
> > > > > > > > > > +Suren, Roman
> > > > > > > > > >
> > > > > > > > > > On Mon, Sep 08, 2025 at 06:00:06PM -0700, Alexei Starovoitov wrote:
> > > > > > > > > > > From: Alexei Starovoitov <ast@kernel.org>
> > > > > > > > > > >
> > > > > > > > > > > Since the combination of valid upper bits in slab->obj_exts with
> > > > > > > > > > > OBJEXTS_ALLOC_FAIL bit can never happen,
> > > > > > > > > > > use OBJEXTS_ALLOC_FAIL == (1ull << 0) as a magic sentinel
> > > > > > > > > > > instead of (1ull << 2) to free up bit 2.
> > > > > > > > > > >
> > > > > > > > > > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > > > > > > > > >
> > > > > > > > > > Are we low on bits that we need to do this or is this good to have
> > > > > > > > > > optimization but not required?
> > > > > > > > >
> > > > > > > > > That's a good question. After this change MEMCG_DATA_OBJEXTS and
> > > > > > > > > OBJEXTS_ALLOC_FAIL will have the same value and they are used with the
> > > > > > > > > same field (page->memcg_data and slab->obj_exts are aliases). Even if
> > > > > > > > > page_memcg_data_flags can never be used for slab pages I think
> > > > > > > > > overlapping these bits is not a good idea and creates additional
> > > > > > > > > risks. Unless there is a good reason to do this I would advise against
> > > > > > > > > it.
> > > > > > > >
> > > > > > > > Completely disagree. You both missed the long discussion
> > > > > > > > during v4. The other alternative was to increase alignment
> > > > > > > > and waste memory. Saving the bit is obviously cleaner.
> > > > > > > > The next patch is using the saved bit.
> > > > > > >
> > > > > > > I will check out that discussion and it would be good to summarize that
> > > > > > > in the commit message.
> > > > > >
> > > > > > Disgaree. It's not a job of a small commit to summarize all options
> > > > > > that were discussed on the list. That's what the cover letter is for
> > > > > > and there there are links to all previous threads.
> > > > >
> > > > > Currently the commit message is only telling what the patch is doing and
> > > > > is missing the 'why' part and I think adding the 'why' part would make it
> > > > > better for future readers i.e. less effort to find why this is being
> > > > > done this way. (Anyways this is just a nit from me)
> > > >
> > > > I think 'why' here is obvious. Free the bit to use it later.
> > > > From time to time people add a sentence like
> > > > "this bit will be used in the next patch",
> > > > but I never do this and sometimes remove it from other people's
> > > > commits, since "in the next patch" is plenty ambiguous and not helpful.
> > >
> > > Yes, the part about the freed bit being used in later patch was clear.
> > > The part about if we really need it was not obvious and if I understand
> > > the discussion at [1] (relevant text below), it was not required but
> > > good to have.
> > > ```
> > >         > I was going to say "add a new flag to enum objext_flags",
> > >         > but all lower 3 bits of slab->obj_exts pointer are already in use? oh...
> > >         >
> > >         > Maybe need a magic trick to add one more flag,
> > >         > like always align the size with 16?
> > >         >
> > >         > In practice that should not lead to increase in memory consumption
> > >         > anyway because most of the kmalloc-* sizes are already at least
> > >         > 16 bytes aligned.
> > >
> > >         Yes. That's an option, but I think we can do better.
> > >         OBJEXTS_ALLOC_FAIL doesn't need to consume the bit.
> > > ```
> > >
> > > Anyways no objection from me but Harry had a followup request [2]:
> > > ```
> > >         This will work, but it would be helpful to add a comment clarifying that
> > >         when bit 0 is set with valid upper bits, it indicates
> > >         MEMCG_DATA_OBJEXTS, but when the upper bits are all zero, it indicates
> > >         OBJEXTS_ALLOC_FAIL.
> > >
> > >         When someone looks at the code without checking history it might not
> > >         be obvious at first glance.
> > > ```
> > >
> > > I think the above requested comment would be really useful.
> >
> > ... and that comment was added. pretty much verbatim copy paste
> > of the above. Don't you see it in the patch?
>
> Haha it seems I am blind, yup it is there.
>
> >
> > > Suren is
> > > fixing the condition of VM_BUG_ON_PAGE() in slab_obj_exts(). With this
> > > patch, I think, that condition will need to be changed again.
> >
> > That's orthogonal and I'm not convinced it's correct.
> > slab_obj_exts() is doing the right thing. afaict.
>
> Currently we have
>
> VM_BUG_ON_PAGE(obj_exts && !(obj_exts & MEMCG_DATA_OBJEXTS))
>
> but it should be (before your patch) something like:
>
> VM_BUG_ON_PAGE(obj_exts && !(obj_exts & (MEMCG_DATA_OBJEXTS | OBJEXTS_ALLOC_FAIL)))
>
> After your patch, hmmm, the previous one would be right again and the
> newer one will be the same as the previous due to aliasing. This patch
> doesn't need to touch that VM_BUG. Older kernels will need to move to
> the second condition though.

Correct. Currently slab_obj_exts() will issue a warning when (obj_exts
== OBJEXTS_ALLOC_FAIL), which is a perfectly valid state indicating
that previous allocation of the vector failed due to memory
exhaustion. Changing that warning to:

VM_BUG_ON_PAGE(obj_exts && !(obj_exts & (MEMCG_DATA_OBJEXTS |
OBJEXTS_ALLOC_FAIL)))

will correctly avoid this warning and after your change will still
work. (MEMCG_DATA_OBJEXTS | OBJEXTS_ALLOC_FAIL) when
(MEMCG_DATA_OBJEXTS == OBJEXTS_ALLOC_FAIL) is technically unnecessary
but is good for documenting the conditions we are checking.